In the manufacture of canned foodstuffs such as meats, vegetables, and beverages, plastic, and wax-coated materials have heretofore achieved only limited use because they are excessively permeable to gases. Attempts to use olefinic resins such as polyethylene and polypropylene have encountered the disadvantage that such resins are excessively permeable to oxygen. The permeation of oxygen into food containers causes undesirable discoloration and depreciation in taste and other sensory qualities of the foodstuff. The poor gas permeability characteristics of polyethylene and polypropylene have resulted in containers fabricated from those resins being rejected in the packaging of oxygen sensitive comestibles.
The manufacture of beverage containers from biaxially oriented polyethylene terephthalate is known in the prior art. As used herein, the terms "polyethylene terephthalate" and "PET" include not only the homopolymer formed by polycondensation of beta-hydroxyethyl terephthalate but also copolyesters containing minor proportions of units that are derived from other glycols and other diacids, such as isophthalic acid.
Biaxially oriented PET containers are strong and have good creep resistance. Containers of relatively thin wall and light weight can be produced from PET. These containers are capable of withstanding, without substantial mechanical deformation over their shelf lives, the pressure generated by carbonated beverages such as soft drinks, beer, and sparkling wines.
Thin-walled PET containers are somewhat permeable to carbon dioxide and oxygen. As a result, PET containers lose their pressurizing carbon dioxide over time and allow transport of oxygen which affects flavor and appearance of the contents. This permeability is more important for small containers having high surface-to-volume ratios than for large containers.
In the prior art, numerous techniques have been devised for reducing the gas and vapor permeability of containers fabricated from PET and other resins. Such techniques include addition of inorganic fillers such as mica, talc, and alumina to the resins; coating the containers with resins having barrier properties; and blending, laminating, or co-extruding the resins with barrier resins. Some barrier resins employed in the prior art for coating, blending, laminating and/or co-extrusion include vinylidene chloride polymers and copolymers such as polyvinylidene chloride, vinylidene chloride/vinyl chloride copolymers, and vinylidene chloride/acrylonitrile copolymers; polyvinyl alcohol and vinyl alcohol copolymers with ethylene; nylon; acrylonitrile/alkyl acrylate copolymers and acrylonitrile/styrene copolymers.
Two resins used for coating various plastic substrates to reduce their gas permeability are vinylidene chloride polymers and polyvinyl alcohol. A latex of polyvinylidene chloride can be applied at the preform stage. However, at this stage the coating must be thick because both coating thickness and body wall thickness are reduced during the subsequent blow molding operation. In practice, the use of polyvinylidene chloride as a coating generally involves application of a plurality of layers with intermediate drying after each layer. The latex can also be applied onto the finished container.
Polyvinyl alcohol is a gas and flavor barrier material having better diffusion resistance than polyvinylidene chloride. However, polyvinyl alcohol is adversely affected by moisture. Among the effects of moisture on polyvinyl alcohol are reduced gas tightness, change in appearance and generally reduced mechanical properties. Accordingly, it is desirable to protect the polyvinyl alcohol layer with a coating of a water-resistant polymer. Moisture sensitivity of this polymer is also reduced by forming a copolymer of vinyl alcohol with ethylene.
The techniques referred to above all generally reduce the permeability of PET and other plastic resins to gases, vapors, and flavors. However, the prior techniques for enhancing barrier properties each suffer from one or more serious disadvantages making them less than entirely suitable for their intended purpose. Such disadvantages include difficulties in handling and co-extruding the barrier resins; increased container bulk and weight; unsatisfactory gas or vapor barrier properties; and high cost.
A principal objective of the present invention is to overcome the above-listed shortcomings of prior art containers by providing a container body coated with a carbon film imparting reduced gas permeability to the body.
A related objective of the invention is to provide a process for producing the film-coated container body.
Additional objects and advantages will become apparent to persons skilled in the art from the following specification and claims.